IonoNet is an INGV (Istituto Nazionale di Geofisica e Vulcanologia) research project developing a multi-static cooperative radar network of Pseudo-Random Code (PRC) ionosondes deployed across Europe. This innovative approach enables oblique soundings to compare ionospheric characteristics over vast distances. The project develops ionosondes for both oblique and vertical soundings, supported by a complete data analysis pipeline. A key feature is the real-time processing capability, which transforms oblique into vertical ionograms, generates electron density profiles using Autoscala program, and can issue ionospheric alerts under disturbed conditions. The system's core innovation is its reliance on Software Defined Radio (SDR) devices for both transmission and reception. This SDRbased architecture provides significant versatility, allowing for flexible configuration of transmission parameters and signal filtering via software, replacing complex traditional hardware. A primary novelty is the network's multi-static capability: a single receiving observatory can simultaneously detect signals from multiple transmitters. This "many-to-one" approach multiplies the observational viewpoints. The system uses unique station identifiers and precise GPS synchronization (GPSDO-GPS Disciplined Oscillator) to manage these complex, simultaneous soundings. Ultimately, this advanced sounding network enables large-scale ionospheric mapping, validation of global models, and the study of local disturbances, space weather impacts, and potential ionosphere-lithosphere coupling. (c) 2025 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Monthly median foF2 values derived from manual and automatic interpretation of the ionograms at the ionospheric observatory of Rome (41.8 degrees N; 12.5 degrees E) over a 17-year period from 2006 to 2022 were considered in this study. The data were analyzed for the hours between 00 and 23 UT, employing two methods when using autoscaled data: one using only exact hour data, as it is made when using manual interpretations, and another incorporating a quarter-hour before and after the hour. ARTIST and Autoscala software were used for the automatic interpretation of the ionograms respectively from the DPS-4 and AIS-INGV ionosondes. Statistical significance of the differences between the medians obtained using automatic and manual values was determined using standard Student's t-tests, and the mean and standard deviation of the differences were also computed for the whole dataset and for each hour of the day separately, along with the corresponding Mean Absolute Errors and Root Mean Square Errors. While the overall differences were not statistically significant in every case, small significant differences (< 0.1 MHz) were obtained during daytime hours for Autoscala, while mainly at night for ARTIST, demonstrating a slight overall underestimation of the monthly medians from automatic interpretation. Only small differences were found between Autoscala and ARTIST medians, which can therefore be considered equivalent, despite slightly superior performance demonstrated by Autoscala, even when including quarters-hour data. These findings indicate that the foF2 values obtained from ARTIST and Autoscala can reliably substitute manual interpretations for monthly medians computation. (c) 2026 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Oblique ionosondes provide an effective means to probe the ionosphere over regions where vertical sounders cannot be deployed, such as oceans or complex terrain. By deploying multiple simple receivers, they also enable a denser spatial sampling of ionospheric conditions. However, extracting ionospheric parameters from oblique ionograms remains challenging due to the long propagation paths and shallow incidence angles of the signals. To address this, verticalization techniques are applied to convert oblique ionograms into equivalent vertical ones, facilitating the retrieval of critical ionospheric parameters. Since 2023, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) has developed and deployed new Software Defined Radio (SDR)-based Pseudo Random Code (PRC) oblique ionosondes, establishing the first nodes of the IonoNet network across Italy. The system operates in a multistatic configuration with separated transmitting and receiving sites in northern, central, and southern Italy, enabling the investigation of ionospheric variability over distances of several hundred kilometres. The Autoscala software is used for real-time ionogram scaling and electron density profile derivation, supporting ionospheric monitoring and alerting applications. The network aims to improve regional ionospheric mapping, validate theoretical models, and explore potential lithosphere-ionosphere coupling processes. (c) 2026 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Understanding the variability of the thermosphere–ionosphere (T–I) system across different conditions is especially important, as its state critically affects the operation and safety of numerous low Earth orbit (LEO) satellites. In the absence of routine thermospheric monitoring, ionosonde measurements and satellite data can be used to retrieve key aeronomic parameters at mid-latitudes during noontime via the THERION (THERmospheric parameters from IONosonde observations) method.This study applies the THERION technique to analyze the T–I response in the European and American longitudinal sector to two recent severe geomagnetic storms (October 2024 and January 2025). Validated ionosonde data from Rome, Juliusruh, Millstone Hill and Eglin were used to assess ionospheric variability and derive thermospheric parameters such as neutral composition, temperature, and wind. Results are compared with outputs from the MSISE00 empirical model, highlighting THERION's improved capability in capturing thermospheric dynamics under storm conditions.Additional datasets—including co-located GNSS-derived TEC, geomagnetic field data from the INTERMAGNET network, and interplanetary/magnetospheric conditions—were integrated to provide a comprehensive view of the events and the unique T-I coupling processes associated with each storm.This study is carried out within the Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 - CUP n. I53D24000060005.

The ionosphere is a crucial region of Earth's atmosphere for radio communication and satellite navigation. Instruments like ionosondes monitor this region and produce ionograms depicting the ionospheric layers. Although automated ionogram scaling tools exist, they often require manual intervention. Recent advancements in deep learning have shown promise in automating ionogram analysis with improved accuracy. However, challenges persist in detecting several ionospheric layers simultaneously and across diverse datasets. This paper compares a traditional U-net with a less complex Convolutional Neural Network architecture and evaluates the performance on a diverse ionogram database covering various sources, locations and periods within the solar cycle. The proposed method is compared to existing deep learning approaches and shows competitive performance in intersection over union (IoU) and recall metrics.

This paper presents a description of a prototype development project for a pulse compression ionosonde designed for vertical surveys using coded pulse radar. The project exploits Software Defined Radio (SDR) devices in an effort to provide a programmable instrument that is more versatile and advanced compared to traditional ionosondes. The programmable ionosonde (AISP) represents an evolution of the AIS (Advanced Ionospheric Sounder) ionosonde, which was entirely designed and built at INGV and is currently operational at several ionospheric observatories. The AISP ionosonde is intended for vertical soundings and primarily consists of an SDR device serving as both transmitter and receiver of Radio Frequency (RF) signals. Additionally, it generates all necessary control signals for the ionosonde's operation, including those for the RF power amplifier and antenna. The SDR device communicates with a Personal Computer (PC) via an Ethernet port, allowing fora wide range of configurations and enhancing the versatility of the ionosonde. Vertical surveys are conducted to map the ionosphere in specific regions, facilitating the validation of theoretical models of the global ionosphere. (c) 2025 The Authors. Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

The ionosphere as vital part of Earth's atmosphere, influences radio communications and satellite navigation. Instruments such as ionosondes are used to monitor this region, generating ionograms that illustrate the ionospheric layers. While automated ionogram scaling tools are available, they frequently require manual intervention. Recent progress in deep learning has demonstrated the potential of automated ionogram scaling. However, the suitability of these techniques on a broad and diverse databases remains still underexplored. This study proposes three robust deep learning-based approaches for the estimation of critical frequencies for a wider variety of ionograms characterized by more general conditions. Despite the increased complexity of the data in relation to state of the art methods in literature, the models achieve high precision and true positive rates, as well as a scaling accuracy that remains within operationally acceptable limits. This work establishes a strong foundation for scalable automatic ionogram analysis suitable for global data.

The nature of the long-term changes in the upper atmosphere morphology at mid-latitude remains a subject of debate, particularly regarding whether these changes are purely driven by geomagnetic and solar activities or whether forcing from the lower atmosphere, such as CO2 variations, may play a role. To contribute to this debate, we investigate the nature of the long-term trends of the ionospheric and thermospheric parameters by leveraging on ionosonde data digitally recorded at the Rome Observatory since 1976. The following parameters have been investigated under sunlit conditions (12:00 Local Time): critical frequency of the F1 layer (foF1); critical frequency of the F2 layer (foF2), atomic oxygen concentration at 300 km ([O]); ratio between atomic oxygen and molecular nitrogen concentrations at 300 km altitude ([O]/[N2]); exospheric temperature (Tex); thermospheric density at 300 km (ρ). The ionospheric parameters are manually scaled from digital ionograms, whereas thermospheric parameters are retrieved using the THERmospheric parameters from IONosonde observations (THERION) method, which utilises ionosonde observations and a physical model of the ionospheric F region. To investigate the influence of the solar and geomagnetic activity on long term variations, we consider the solar radio flux at 10.7 cm (F10.7) and the geomagnetic disturbance index Ap. To identify the various frequency/period components of the time series under consideration and identify the trends, we leverage the high scale/time resolution offered by the Fast Iterative Filtering (FIF) algorithm. A regression analysis of thermosphere/ionosphere parameters against geomagnetic/solar activity indices has then been conducted to investigate the drivers of long-term variability. Our findings reveal that the identified trends are predominantly controlled by external drivers, particularly long-term solar and geomagnetic activity variations.. The adopted methodology, based on regression modelling, demonstrates that variability in F10.7 and Ap accounts for nearly all of the observed changes, with the exception of atomic oxygen ([O]), which displays a slightly higher unexplained variability (~7%). The inclusion of CO2 concentration as an additional driver improves the regression model for [O]. However, the effect remains statistically limited, indicating that the impact of CO2 on thermospheric cooling might be of little significance. Further studies with extended time series are necessary to better quantify this relationship and evaluate its importance. These results highlight the predominant influence of solar and geomagnetic activity in determining upper atmosphere long-term trends at mid-latitudes.

On 8 May 2024, the solar active region AR13664 started releasing a series of intense solar flares. Those of class X released between 9 and 11 May 2024 gave rise to a chain of fast Coronal Mass Ejections (CMEs) that proved to be geoeffective. The Storm Sudden Commencement (SSC) of the resulting geomagnetic storm was registered on 10 May 2024 and it is, to date, the strongest event since November 2003. The May 2024 storm, named hereafter Mother’s Day storm, peaked with a Dst of –412 nT and stands out as a “standard candle” storm affecting modern era technologies prone to Space Weather threats. Moreover, the recovery phase exhibited almost no substorm signatures, making the Mother’s Day storm as a perfect storm example. Despite the plethora of notable near Earth environment modifications that are still under investigation, in this paper we concentrate on the Space Weather effects over the Mediterranean sector, with a focus on Italy. In fact, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) manages a dense network of GNSS receivers (including scintillation receivers), ionosondes and magnetometers in the Mediterranean area, which facilitated for a detailed characterization of the modifications induced by the storm. Concerning the geomagnetic field, observatories located in Italy recorded a SSC with a rise time of only 3 minutes and a maximum variation of around 600 nT. The most notable ionospheric effect following the arrival of the disturbance was a significant decrease in plasma density on 11 May, resulting in a pronounced negative ionospheric storm registered on both the critical F2-layer frequency (foF2) and the Total Electron Content (TEC). Another negative effect was recorded on 13 May, while no signatures of composition changes and, specifically, to a decrease of the [O]/[N ] ratio. The IRI UP IONORING 2 data-assimilation procedure, recently developed to nowcast foF2 over Italy, proved to be quite reliable during this extreme event, being characterised just by an overestimation during the main phase of the storm, when the electron density and the height of the F region decreased and increased, respectively. Relevant outcomes of the work relate to the Rate Of TEC change Index (ROTI), which shows unusually high spatially distributed values on the nights of 10 and 11 May. The ROTI enhancements on 10 May might be linked to Stable Auroral Red (SAR) arcs and an equatorward displacement of the main ionospheric trough. Instead, the ROTI enhancements on 11 May might be triggered by a joint action of low-latitude plasma pushed poleward by the pre-reversal enhancement (PRE) in the post-sunset hours and wave-like perturbations propagating from the North. Furthermore, the storm generated immediate attention of the general public to Space Weather effects, including mid-latitude visible phenomena like SAR arcs. This paper outlines the report of the Space Weather Monitoring Group (SWMG) of the INGV Environment Department and its effort to disseminate information about this exceptional event.

A comparison of three types of ionosonde data from Europe during an interplanetary coronal mass ejection (ICME)- and a corotating interaction region (CIR)-driven geomagnetic storm event is detailed in this study. The selected events are 16–20 March 2015 for the ICME-driven storm and 30 May to 4 June 2013 for the CIR-driven one. Ionospheric data from three European ionosonde stations, namely Pruhonice (PQ), Sopron (SO) and Rome (RO), are investigated. The ionospheric F2-layer responses to these geomagnetic events are analyzed with the ionospheric foF2 and h’F2 parameters, the calculated deltafoF2 and deltahF2 values, the ratio of total electron content (rTEC) and Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) satellite Global Ultraviolet Imager (GUVI) thermospheric [O]/[N2] measurement data. The storm-time and the quiet-day mean values are also compared, and it can be concluded that the quiet-day curves are similar at all the stations while the storm-time ones show the latitudinal dependence during the development of the storm. As a result of the electron density comparison, during the two events, it can be concluded that the sudden storm commencement (SSC) that characterized the ICME induced a traveling atmospheric disturbance (TAD) seen in the European stations in the main phase, while this is not seen in the CIR-driven ionospheric storm, which shows a stronger and more prolonged negative effect in all the stations, probably due to the season and the depleted O/N2 ratio.

The comparison of three types of ionosonde data from Europe during an Interplanetary Coronal Mass Ejection (ICME) and a Stream Interaction Regions (SIRs) / Corotating Interaction Regions (CIRs) -driven geomagnetic storm event is detailed in this study. The selected events are 16-20 March 2015 for the ICME-driven and 30 May to 04 June 2013 for the SIR/CIR-driven one. Ionospheric data from three European ionosonde stations, namely Pruhonice (PQ), Sopron (SO) and Rome (RO), are investigated. The ionospheric F2-layer responses to these geomagnetic events are analyzed with the ionospheric foF2 and h’F2 parameter, the calculated deltafoF2 and deltahF2 values, ratio of Total Electron Content (rTEC) and Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) satellite Global Ultraviolet Imager (GUVI) thermospheric [O]/[N2] measurement data. The storm-time and the quiet day mean values are also compared, and it can be concluded that the quiet day curves are similar at all stations while the storm-time ones showed the latitudinal dependence during the development of the storm. As a result of the electron density comparison, during the two events it can be concluded that SSC that characterized the ICME induced a Travelling Atmospheric Disturbance (TAD) seen in the European stations, while this is not in the SIR/CIR-driven ionospheric storm, which showed a stronger and more prolonged negative effect in all stations probably due to the season.

Several empirical formulations used over time to estimate the fundamental ionospheric parameter hmF2 have been compared in this study. These are the first formulation proposed by Shimazaki (1955) (SHI-1955) as a function of the propagation parameter M(3000)F2, the more accurate BSE-1979 formula proposed by Bilitza et al. (1979) and firstly adopted by the International Reference Ionosphere (IRI) model, and the newest Altadill-Magdaleno-Torta-Blanch (AMTB-2013) (Altadill et al., 2013) and SHU-2015 (Shubin, 2015) mod-els, obtained with a different approach with no explicit dependence on any ionospheric parameter and added as alternative options in the IRI-2016. The evaluation of the accuracy of the available formulation is performed by comparing the modeled values of hmF2 with those simultaneously obtained with independent measurements from the Incoherent Scatter Radar (ISR) installed at the Millstone Hill ionospheric station. The database considered consists of 3626 measurements, thus allowing the evaluation of the results for different helio-geophysical conditions. SHI-1955 and BSE-1979 formulations are evaluated also using input data manually scaled from ionograms recorded at the same location, with the aim of evaluating their accuracy when updated with validated data rather than modeled ones. The SHU-2015 is confirmed the best option in any condition, while AMTB-2013 turns out to perform poorly during night, when SHI-1955 and BSE-1979 fed by validated data can be used for trend analyses due to the high correlation with ISR data. Despite this, BSE-1979 performs better with modeled parameters as input, in terms of RMSE and mean deviation from ISR data. The use of SHI-1955 with CCIR-modeled M(3000)F2 is discouraged under daytime conditions even for long trend analyses.(c) 2023 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

We have used 2014–2017 data from the eight receiving stations of the Japan very low frequency (VLF) monitoring network. The nighttime data of the signals of the JJI transmitter on Kyushu Island, excited VLF electromagnetic waves (EMWs) in the Earth-Ionosphere waveguide (EIWG) had been processed. The wavelet transform with a preliminary detrending, to exclude influence of daily variations, has been applied. We have observed ultra-low frequency (ULF) modulation of VLF EMW spectra in the EIWG. We therefore concluded that modulating oscillations with periods of 4 minutes belong to the acoustic branch of acoustic-gravity waves (AGWs) in the Earth–Thermosphere waveguide; modulation of VLF with periods of 6–7 minutes corresponds to global evanescent/reactive Brunt–Väisälä AGW oscillations; the oscillations with periods 20–60 min and ~3 hours may characterize evanescent/reactive Lamb gravity wave mode of AGW [1]. The appearance of the combination frequency of VLF EMW and ULF AGW is likely due to the following effects: (1) the drag of charged plasma particles by ULF AGWs jointly with the background of VLF electron density disturbances and (2) the motion of charged plasma particles in the VLF EMW field jointly with the background of ULF changes in the plasma concentration caused by AGWs.The theory [2,3] is extended to the excitation of ionospheric Schumann resonator (SR) [4] and ionospheric Alfvén resonator (IAR) in the ULF range. It is shown that IAR oscillations with a high quality factor (for geophysical resonators) (>10) can be excited in the SR range. The features of the excited ULF and VLF modes associated with the modification of the ionosphere as a result of the powerful eruption of the Hunga-Tonga volcano are under consideration [5,6].A ULF model of perturbations in the atmosphere-ionosphere with a boundary transition from dynamic to static limit is developed and the preliminary results of the corresponding modelling will be presented. This ensures the "recovery" of magnetostatic disturbances "lost" in most of previous models of the atmospheric electrical circuit, important for understanding the mechanisms of seismo-ionospheric coupling, volcano-ionospheric coupling and influences of the other natural hazards on the ionosphere and ionospheric monitoring of the natural hazards.[1] Rapoport et al. Sensors 22, 10.3390/s22218191, 2022; [2] Grimalsky et al. JEMAA 2012, 4, 192-198 ; [3] Yutsis V. et al. Atmosphere 2021, 12, 801 ; [4] Nickolaenko and Rabinovich Space Res. 1982, XX, 67-88 ; [5] Astafyeva et al. GRL, 2022 ; [6] D’Arcangelo et al., Rem. Sens., 14, 3649, 2022.This research was partially funded by the National Science Centre, Poland, grant No. 970 2022/01/3/ST10/00072

The “Monitoring Earth’s Evolution and Tectonics” (MEET) project is funded by the Italian “Piano Nazionale di Ripresa e Resilienza (PNRR) – Next Generation EU”, approved by the Ministry of University and Research. MEET is in the framework of the Research Infrastructure European Plate Observing System “EPOS” and aims to strengthen the observational systems dedicated to discovering the Earth’s dynamics, focusing on the Italian territory, particularly those regions more affected by natural hazards.

On 3 November 2021, an interplanetary coronal mass ejection impacted the Earth’s magnetosphere leading to a relevant geomagnetic storm (Kp = 8-), the most intense event that occurred so far during the rising phase of solar cycle 25. This work presents the state of the solar wind before and during the geomagnetic storm, as well as the response of the plasmasphere–ionosphere–thermosphere system in the European sector. To investigate the longitudinal differences, the ionosphere–thermosphere response of the American sector was also analyzed. The plasmasphere dynamics was investigated through field line resonances detected at the European quasi-Meridional Magnetometer Array, while the ionosphere was investigated through the combined use of ionospheric parameters (mainly the critical frequency of the F2 layer, foF2) from ionosondes and Total Electron Content (TEC) obtained from Global Navigation Satellite System receivers at four locations in the European sector, and at three locations in the American one. An original method was used to retrieve aeronomic parameters from observed electron concentration in the ionospheric F region. During the analyzed interval, the plasmasphere, originally in a state of saturation, was eroded up to two Earth’s radii, and only partially recovered after the main phase of the storm. The possible formation of a drainage plume is also observed. We observed variations in the ionospheric parameters with negative and positive phase and reported longitudinal and latitudinal dependence of storm features in the European sector. The relative behavior between foF2 and TEC data is also discussed in order to speculate about the possible role of the topside ionosphere and plasmasphere response at the investigated European site. The American sector analysis revealed negative storm signatures in electron concentration at the F2 region. Neutral composition and temperature changes are shown to be the main reason for the observed decrease of electron concentration in the American sector.

As it is known, Space Weather (SWx) phenomena can have dramatic impact on satellite navigation and HF radio communication systems, being also responsible for increases on radiation levels at flight altitudes. For this reason, in recent years the International Civil Aviation Organization (ICAO) has been showing great interest in operational SWx services for aviation purposes in these three domains. Four global SWx centers have been then appointed since November 2019 by ICAO to provide real-time SWx advisories for aviation users. In particular, HF COM conditions are assessed by monitoring the F2-layer critical frequency foF2 or the MUF(3000) ionospheric characteristic (MUF = Maximum Usable Frequency), the latter representing the highest HF radio frequency that can be used for communications over a standard distance of 3000 km via F2-layer ionospheric reflection. As one of the designed SWx centers, several key operational 24/7 products for HF COM conditions assessment have been developed within PECASUS (Partnership for Excellence in Civil Aviation Space weather User Services). Nowcasting and forecasting (1-24hr) maps over Europe of MUF(3000) and its ratio with respect to a background level are then developed by INGV, as a PECASUS partner. The MUF(3000) nowcasting uses all the available real-time ionosonde measurements in different locations in order to upgrade IRI-CCIR-based background maps, and Ordinary Kriging method for spatial interpolation. The MUF(3000) modeling performance was assessed comparing predicted values to measured ones over two test stations during strong geomagnetic storm periods, obtaining an overall RMSE < 2 MHz at both stations. The MUF(3000) predicted 1-24 hours ahead depends on foF2 and M(3000) ionospheric parameters: EUROMAP forecasting model and IRI model are used for the former and the latter, respectively. The method has been applied to Europe where there are ionospheric stations with long (for some solar cycles) historical data and current real-time foF2 observations. A mapping procedure applied to the European stations provides MUF(3000) short-term prediction over the whole area. The application of these methods to storm events occurred after November 2019 is here presented, in order to study the ionospheric conditions they provide when HF COM advisories are expected to be issued.

Automatic ionogram interpretation methods developed for real-time ionospheric monitoring can be applied in retrospective studies to analyze large quantities of data. The Autoscala software, implemented for such a purpose, includes a routine for automatic detection of diffused echoes known as spread F, which appear in ionograms due to the presence of ionospheric irregularities along the radio signal path. The main objective of this routine is to reject bad quality ionograms. This new capability was used in a climatological study including a large number of ionograms recorded at the low-latitude ionospheric station of Tucumán (26.9° S, 294.6° E, magnetic latitude 15.5° S, Argentina). The study took into account different levels of geomagnetic and solar activity from 2012 to 2020. The results demonstrate the capability of Autoscala to capture the main signature characteristics of spread F and the temporal evolution of the ionosphere peak heigh hmF2, capturing the post-sunset plasma surge that precedes development of spread F. Maximum occurrence of spread F is observed in local summer, with a tendency to shift before midnight with increasing solar activity. Other new climatological details that emerged from the study are illustrated and briefly discussed, dealing with connection with geomagnetic activity, and morning hmF2 behavior after extremely marked nighttime spread F occurrence.